The present invention relates to fiber-optic communications networks, and more particularly, to optical network equipment such as optical amplifiers in which pump powers are controlled to suppress gain transients while maintaining stable operation.
Fiber-optic networks are used to support voice and data communications. In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber.
Optical amplifiers are used in fiber-optic networks to amplify optical signals. For example, optical amplifiers may be used to amplify optical data signals that have been attenuated by fiber spans and components in fiber-optic links. A typical amplifier may include erbium-doped fiber coils that are pumped with diode lasers.
Optical amplifiers should have low noise figures to ensure satisfactory amplification of the optical signals. Optical amplifiers should also have stable output powers to avoid signal power instabilities. Gain transients should be well controlled, so that amplifier gain remains constant under fluctuating input power conditions.
It is an object of the present invention to provide optical network equipment such as optical amplifiers that suppress gain transients due to fluctuations in input power.
It is also an object of the present invention to provide optical network equipment such as optical amplifiers that exhibit stable output powers while operating under low input power conditions.
These and other objects of the invention are accomplished in accordance with the present invention by providing optical amplifiers and other optical network equipment for use in fiber-optic communications links in fiber-optic networks. The fiber-optic links may be used to carry optical data signals associated with wavelength-division-multiplexing channels.
The equipment may include optical gain stages. The gain stages may be based on optically pumped fiber such as rare-earth-doped fiber. Laser diode pumps may be used to optically pump the fiber. Input and output taps may be used to monitor the optical input power to the equipment and the optical gain of the gain stages. The pump power of the pumps in the gain stages may be adjusted in real time based on the measured optical powers to suppress gain transients that would otherwise arise due to fluctuations in the input power to the equipment. The pump power of the pumps in the gain stages may be adjusted so that it tracks the measured signal input power. This helps to maintain the gain of the amplifier at a constant level during input power fluctuations.
Copumping and counterpumping arrangements may be used to pump one or more coils of the fiber in the gain stages. Some laser diodes may have a tendency to be unstable and noisy when operated at powers just above the lasing threshold due to mode partition noise. The stability of the amplifier gain and output power may be improved at low input powers by avoiding such unstable operating regimes.
For example, when a coil is pumped by two lasers, the powers of the lasers may be adjusted during-transient control operations so that at low input powers one laser takes over from the other. With this type of arrangement, the power of the surviving laser is made relatively higher, so that it operates well above the noisy operating regime. The dropped laser produces no output power, so that it does not contribute noisy pump power to the coil.
Pump power splitting arrangements, remnant pump arrangements, and proper pump power wavelength selection may also be used to improve the performance of the amplifier under low input power conditions.